Zealandia: The Drowned Continent of the South

It's a fact every student learns in school: that Earth has seven continents: Europe, Asia (really Eurasia), Africa, North America, South America, Australia, and Antarctica.  As it turns out, there's an eighth one—the drowned continent of Zealandia. Geologists confirmed its status early in 2017, after years of mystery about just what was going on deep beneath the waves of the South Pacific near New Zealand.

The mystery was tantalizing: continental rocks where none should exist, and gravity anomalies surrounding a large chunk of underwater territory. The culprit in the mystery? Huge slabs of rock buried deep beneath the continents. These huge conveyor-belt-like subsurface chunks of rock are called tectonic plates. Their motions of those plates have substantially changed all the continents and their positions since the time Earth was born, some 4.5 billion years ago.

Now it turns out they also caused a continent to disappear. That's the story geologists are uncovering with the revelation that New Zealand and New Caledonia in the South Pacific are really the highest points of the long-lost continent Zealandia. It's a tale of long, slow motions over millions of years that sent much of Zealandia plummeting below the waves, and the continent wasn't even suspected to exist until the twentieth century.

THE STORY OF ZEALANDIA

This long-lost continent, sometimes also called Tasmantis, formed very early in Earth's history. It was part of Gondwana, a huge supercontinent that existed as early as 600 million years ago. As it, too, was carried by tectonic plates, it eventually merged with another primordial continent called Laurasia to form an even larger supercontinent called Pangaea.

Zealandia's watery fate was sealed by the motions of two tectonic plates that lay beneath it: the southernmost Pacific Plate and its northern neighbor, the Indo-Australian plate. They were slipping past each other a few millimeters at a time each year, and that action slowly pulled Zealandia away from Antarctica and Australia beginning some 85 million years ago. The slow motions caused Zealandia to sink, and by the late Cretaceous period (some 66 million years ago) much of it was underwater. Only New Zealand, New Caledonia and a scattering of smaller islands remained above sea level.

ZEALANDIA'S GEOLOGY

The motions of the plates that caused Zealandia to sink continue to shape the underwater geology of the region into sunken regions called grabens and basins. Volcanic activity also occurs throughout the areas where one plate is subducting (diving under) another. Where the plates compress against each other, the Southern Alps exist where uplifting motion has sent the continent upward. This is similar to the formation of the Himalaya Mountains where the Indian Subcontinent meets the Eurasian plate.

Zealandia's oldest rocks date back to the Middle Cambrian period (about 500 million years ago).

These are mainly limestones, sedimentary rocks made of the shells and skeletons of marine organisms. There is also some granite, an igneous rock made up of feldspar, biotite, and other minerals, that dates back to about the same time. Geologists continue to study rock cores in the hunt for older materials and to relate Zealandia's rocks with its former neighbors Antartica and Australia. The older rocks found so far are underneath layers of other sedimentary rocks that show evidence of the breakup that began to sink Zealandia millions of years ago. In the regions above water, volcanic rocks and features are evident throughout New Zealand and some of the remaining islands.

HOW DID GEOLOGISTS FIND ZEALANDIA?

The story of Zealandia's discovery is almost like a geological puzzle, with the pieces coming together over many decades.

Scientists knew of the submerged areas of the region for many years, dating back to the early part of the 20th century, but it was only about twenty years ago that they began to consider the possibility of a lost continent. Detailed studies of the ocean surface in the region showed that the crust was different from other ocean crust. Not only was it thicker than oceanic crust, the rocks brought up from the ocean bottom and drilling cores were not oceanic crust rock. They were the continental type. How could this be, unless there was actually a continent hidden beneath the waves?

Then, in 2002, a map taken using satellite measurements of the gravity of the region showed revealed the rough structure of the continent. Essentially, the gravity of oceanic crust is different from that of continental crust and that can be measured by satellite. The map showed a definite difference between the regions of deep-ocean bottom and Zealandia. That was when geologists began to think that a missing continent had been found. Further measurements of rock cores, subsurface studies by marine geologists, and more satellite mapping influenced geologists to consider that Zealandia actually is a continent. The discovery, which took decades to make, was announced early in 2017 when a team of geologists announced that Zealandia was officially a continent.

WHAT'S NEXT FOR ZEALANDIA?

The lost continent is rich with natural resources, which have been exploited for some time. It has unique biological populations, as well as mineral deposits that are actively under development. For geologists and planetary scientists, the area holds many clues to the past of our own planet and may help scientists understand landforms seen on other worlds in the solar system.